I've noted that there is a deafening silence from qualified metallurgists, so as a chartered mechanical engineer, I'll advise what little I know on this subject.? NO specialist
is going to proffer free advice or any advice that allows you to proceed safely without a.) knowledge of what the steel consists of, b.) the application, including stresses, safety implications etc. and c.) professional indemnity insurance, so what follows
will deliberately be vague while explaining the difficulties against which you are up.
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The standard advice to welders who want to weld high strength steels is just don't.? If steel is heat treated, it implies that there was some objective in heat treating it,
such as hardening, and therefore it is at least medium if not high strength.
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That instruction can be qualified to some extent by someone with a higher level of qualification.
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Steel that is nominally not hardenable by heat treatment is close to pure iron and is referred to as mild steel.? This is used for structures, boilers, etc. and the whole point
is that the steel and its weld are ductile.
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To make it hardenable, various alloying elements are added, the most basic of which is carbon.? This allows the formation of iron carbide, which is very hard.? To achieve maximum
hardness, this requires a very quick quench, so you cannot through harden thick sections.? To slow down the quench requirements and allow thicker sections to be through hardened various other alloying elements can be added, such as Chromium, Vanadium, Cobalt,
Nickel, Manganese etc. ?(Manganese is in virtually all steel anyway, just as a means of mopping up impurities like Phosphorous and Sulphur.? In higher quantities than required to balance the impurities, it leads to extreme work hardening.)
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In the periodic table of elements, Hydrogen is grouped with the metals.? It is soluble in liquid iron.? For reasons I don't understand, it doesn't cause so much of a problem
in thin mild steel, but in hardenable steel, or even thick mild steel, it comes out of solution during the solidification process and congregates at the grain boundaries, so the grains are not stuck together as well, thereby causing gross lack of strength.
?There is a limit to the % carbon in the steel beyond which you just cannot successfully weld.? Each alloying element is given a carbon equivalent, usually less than 1.? You add all those up in proportion to their % in the composition and add them to the carbon
%.? At a lower figure than the limit, welding can be successful but only if a suitable procedure is devised and followed.? This typically includes the exclusion of hydrogen (moisture, which is why rods are baked for extensive periods), preheating the job for
a period to a specified temperature, maintaining a certain minimum temperature during the welding, maintaining a specified temperature for a significant period after the welding to allow the hydrogen to diffuse out of the job, which it does only slowly, and
finally cooling very slowly.? This is fine if you don't mind losing all the high strength properties as a result of the slow cooling.? Never gone further than this myself, but it may then be possible to reharden the whole job in the normal way.
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Very sorry, but I feel you're going to have to pay a pro for a procedure if the job is critical, or try some of the above and test it if a few failures are of little consequence.
?There should be info on t'interweb thingy about carbon equivalents and limits.
?
Eddie
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------ Original Message ------
From: ask_derrick=[email protected]
To: [email protected]
Sent: Wednesday, May 15th 2024, 19:25
Subject: [SouthBendLathe] Anyone in the Group a Degreed Metallurgist?
I am working on a project and need some advice on welding heat treated materials.
Ask_derrick@...
Nick Andrews